1. Field of the Invention
The present invention relates to a process for producing a retainer for a constant velocity joint, possessing improved machinability and flexural strength, as a boron-containing automobile component.
2. Background Art
A steel corresponding to SCr 415 (see JIS G 4104 (1979)) or SCM 415 (see JIS G 4105 (1979)) has hitherto been carburized, quenched, and tempered before use for a retainer for an automobile constant velocity joint (CVJ). In recent years, however, a reduction in weight of components is necessary for realizing an improvement in fuel consumption from the viewpoint of global environmental problems. The reduction in weight of the retainer requires the use of materials possessing excellent flexural strength. In the invention disclosed in Japanese Patent Laid-Open No. 53169/1997, a high-strength CVJ carburized and quenched component is provided using a boron-containing case hardening steel. On the other hand, the machinability should not be sacrificed from the viewpoint of reducing the production cost. In the above steel, the addition of alloying elements such as silicon (Si) and molybdenum (Mo) is indispensable, and, thus, a deterioration in machinability is unavoidable. This has led to the need to develop a retainer for a constant velocity joint possessing improved flexural strength without sacrificing the machinability.
It is an object of the present invention to provide a retainer using a steel corresponding to SCr 415 or SCM 415 for a constant velocity joint possessing improved flexural strength properties without sacrificing machinability for automobiles or the like.
According to one aspect of the present invention, there is provided a process for producing a retainer for a constant velocity joint having improved machinability and flexural strength properties, said process comprising the steps of:
providing a steel comprising by weight carbon (C): 0.10 to 0.25%, silicon (Si): 0.03 to 0.15%, manganese (Mn): 0.20 to 0.60%, sulfur (S): 0.003 to 0.030%, chromium (Cr): 1.00 to 1.50%, titanium (Ti): 0.05 to 0.20%, boron (B): 0.0005 to 0.0050%, and nitrogen (N): not more than 0.01% with the balance consisting of iron (Fe) and inevitable impurities;
hot forging the steel at a heating temperature of 1000 to 1100xc2x0 C. to prepare a forged product having a Rockwell hardness of not more than 88 HRB;
machining and optionally piercing the forged product;
carburizing, quenching, and tempering the machined steel to attain an effective case depth of 0.4 to 0.9 mm, said effective case depth being defined as the distance from the surface of the steel to the position of the steel in its depth direction where the Vickers hardness is HV 550.